Medium Access Protocols

Transcription

1 Medium Access Protocols

2 Summary of MAC protocols What do you do with a shared media? Channel Partitioning, by time, frequency or code Time Division,Code Division, Frequency Division Random partitioning (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technologies (wire), hard in others (wireless) CSMA/CD used in Ethernet Taking Turns polling from a central cite, token passing

4 LAN Addresses and ARP 32-bit IP address: network-layer address used to get datagram to destination network LAN (or MAC or physical) address: used to get datagram from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs) burned in the adapter ROM

11 Ethernet Frame Structure (more) Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match Type/length: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply dropped

12 Ethernet: uses CSMA/CD A: sense channel, if idle then { transmit and monitor the channel; If detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} } else {wait until ongoing transmission is over and goto A}

13 Ethernet s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Exponential Backoff: Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K x 512 bit transmission times after second collision: choose K from {0,1,2,3} after ten or more collisions, choose K from {0,1,2,3,4,,1023}

20 Interconnecting LANs Q: Why not just one big LAN? Limited amount of supportable traffic: on single LAN, all stations must share bandwidth limited length: specifies maximum cable length large collision domain (can collide with many stations) limited number of stations: have token passing delays at each station

21 Hubs Physical Layer devices: essentially repeaters operating at bit levels: repeat received bits on one interface to all other interfaces Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

22 Hubs (more) Each connected LAN referred to as LAN segment Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN Hub Advantages: simple, inexpensive device Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions extends maximum distance between node pairs (100m per Hub)

23 Hub limitations single collision domain results in no increase in max throughput multi-tier throughput same as single segment throughput individual LAN restrictions pose limits on number of nodes in same collision domain and on total allowed geographical coverage cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)

24 Bridges Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination Bridge isolates collision domains since it buffers frames When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit

25 Bridges (more) Bridge advantages: Isolates collision domains resulting in higher total max throughput, and does not limit the number of nodes nor geographical coverage Can connect different type Ethernet since it is a store and forward device Transparent: no need for any change to hosts LAN adapters

30 Bridge Filtering filtering procedure: if destination is on LAN on which frame was received then drop the frame else { lookup filtering table if entry found for destination then forward the frame on interface indicated; else flood; /* forward on all but the interface on which the frame arrived*/ }

31 Bridge Learning: example Suppose C sends frame to D and D replies back with frame to C C sends frame, bridge has no info about D, so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D

33 Spanning Tree The learning bridge fails when the network topology has a loop. Why? Loops are not necessarily bad. They provide redundancy that can be used to recover from failures To handle loops, bridges implement the spanning tree algorithm. The spanning tree algorithm imposes a logical tree over the physical topology Data is only transferred along links that belong to the spanning tree

35 Spanning Tree Algorithm (contd.) Bridges exchange configuration messages called CBPDU s(configuration Bridge Protocol Data Unit) id for bridge sending the message id for what the sending bridge believes to be root bridge distance (hops) from sending bridge to root bridge Each bridge records the current best configuration message for each port Initially, each bridge believes it is the root

36 Spanning Tree Algorithm (contd.) When a bridge learns that it is not the root it stops generating configuration messages in steady state, only root generates configuration messages When the bridge learns that it is not the designated bridge, it stops forwarding configuration messages in steady state, only designated bridges forward config messages Root continues to periodically send config messages If any bridge does not receive successive config messages, it starts generating config messages claiming to be the root This is used to recover from root failure

37 Limitations of Bridges Do not scale spanning tree algorithm does not scale single large broadcast domains do not scale Do not accommodate heterogeneity Bridges support ethernet to ethernet, ethernet to and to Caution: beware of transparency Applications that assume that they are executing on a single LAN will fail. Latency increases in large LANs, so does jitter

39 Routers vs. Bridges Bridges + and - + Bridge operation is simpler requiring less processing bandwidth - Topologies are restricted with bridges: a spanning tree must be built to avoid cycles - Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)

Chapter 5 Link Layer and LANs A note on the use of these ppt slides: All material copyright 1996-2007 J.F Kurose and K.W. Ross, All Rights Reserved Computer Networking: A Top Down Approach 4 th edition.

Chapter 5 Link Layer and LANs A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you can add, modify, and

Lecture 4b Local Area Networks and Bridges Ethernet Invented by Boggs and Metcalf in the 1970 s at Xerox Local area networks were needed to connect computers, share files, etc. Thick or Thin Ethernet Cable

Switching and Forwarding Reading: Chapter 3 1/30/14 1 Switching and Forwarding Next Problem: Enable communication between hosts that are not directly connected Fundamental Problem of the Internet or any

CSCI-1680 Link Layer Wrap-Up Rodrigo Fonseca Based partly on lecture notes by David Mazières, Phil Levis, John Jannotti Administrivia Homework I out later today, due next Thursday Today: Link Layer (cont.)

Redes de Computadores Medium Access Control Manuel P. Ricardo Faculdade de Engenharia da Universidade do Porto 1 » How to control the access of computers to a communication medium?» What is the ideal Medium

Chapter 5 Link Layer and LANs Computer Networking: A Top Down Approach Featuring the Internet, A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students,

CH : 15 LOCAL AREA NETWORK OVERVIEW P. 447 LAN (Local Area Network) A LAN consists of a shared transmission medium and a set of hardware and software for interfacing devices to the medium and regulating

Data Link Protocols TCP/IP Suite and OSI Reference Model The TCP/IP protocol stack does not define the lower layers of a complete protocol stack In this lecture, we will address how the TCP/IP protocol

Module 4 Data Link Layer CS655! 4-1! Please note: Most of these slides come from this book. Note their copyright notice below! A note on the use of these ppt slides: We re making these slides freely available

2. LAN Topologies Two basic categories of network topologies exist, physical topologies and logical topologies. The physical topology of a network is the cabling layout used to link devices. This refers

Cisco 200-125 Cisco Certified Network Associate (CCNA) http://killexams.com/pass4sure/exam-detail/200-125 Question: 769 Refer to exhibit: Which destination addresses will be used by Host A to send data

Chapter 5 Link Layer A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations; and can

Chapter 5 Link Layer A note on the use of these ppt slides: We re making these slides freely available to all (faculty, students, readers). They re in PowerPoint form so you see the animations; and can

Data Communications Connecting Devices Connecting Devices Networks do not normally operate in isolation. They are connected to one another or to the Internet. To connect LANs, or segments of LANs, we use

The MAC Layer Jean Yves Le Boudec Fall 2012 1 Contents 1. MAC as Shared Medium : The Ethernet Myth and the WiFi Reality 2. MAC as interconnection at small scale : Why Ethernet became a point to point technology